Lasers are widely used today in fiber and free space segments for high data rate communication links, remote sensing applications (LIDAR) and more. In these applications the modulated light signal is modulated using electro-optical modulators and demodulated using, usually, electro-optical receiving devices.
In optical communications the modulation scheme commonly used is On-Off Keying (OOK), as illustrated in FIG. 1(a), where only the power of the light is modulated. Alternative modulation schemes include Phase Shift Keying (PSK) (FIG. 1(b)), where the data is encoded in the phase of the signal. In RF communications more advanced modulation schemes are used, such as Quadrature Phase Shift Keying) (QPSK) (FIG. 1(c)) and Quadrature Amplitude Modulation (QAM), (FIG. 1(d)).
By using such communication schemes, for example, in optical communication systems, the capacity and link performance can be greatly enhanced in comparison with the direct detection schemes. In LIDAR, which is the extension of radar to the optical domain, the required shaping of the pulses can be achieved, such as chirped pulses, Barker coding, etc.
Such modulating formats as PSK (for example, BPSK and QPSK) were used mostly in the coherent communication systems (see, for example, L. G. Kazovsky, “Phase- and Polarization-Diversity Coherent Optical Techniques”, J. of Lightwave Technology, Vol 7, N2, pp. 279-292, 1989.). The majority of the work in this field was made by implementing non-integrated solutions, i.e. various optical components such as amplitude and phase modulators connected by optical fibers. Such communication schemes were abandoned in the late 1980's and are still not implemented due to their complexity and high cost.
For these applications and others, the light should be modulated both in amplitude and phase, essentially with a complex modulation signal. The cost is increased complexity of receivers in coherent detection schemes. The main problem is related to the development of compact, reliable, and low-cost receivers for such advanced modulating schemes.
At the receiver the received optical signal is mixed with the local oscillator signal by an optical interface that is usually based on one or more optical hybrids, such as directional hybrids, polarization splitters, and 90-degrees balanced hybrids. At the output from the optical interface, the optical field is converted into electric currents by one or more PIN photodiodes.
If the local oscillator and the received optical carrier have the same frequency, the electric currents provided by the photodiodes are baseband signals and the receiver is of the homodyne type. Respectively, if the local oscillator and the received optical carrier have different frequencies, the electric currents are shifted to the intermediate frequency (IF).
The present invention relates generally to the integrated phase diversity optical receiver designated to detect the optical signal, to mix it with another optical signal, to transform the signal into electrical domain for further processing. In-phase and in-quadrature detection can be applied to coherent optical receiver as a technique to measure simultaneously the phase and amplitude of the optical field, see for example, N. G. Walker, J. E. Carroll, “Simultaneous phase and amplitude measurements on optical signals using a multi port junction”, Electronics Letters, Vol. 20, N23, 981-983, 1984 and T. G. Hodkinson, et al., “Demodulation of optical DPSK using in-phase and quadrature detection”, Electronics Letters, Vol. 21, N19, 867-868, 1985.
Optical devices currently available are based on nonintegrated and/or semi-integrated solutions, i.e. optical fibers or optical fiber-based components are used for connecting of various electro-optical components and/or splitting/combining the optical signals. There are no completely planar integrated solutions for the device that are capable to provide an arbitrary format demodulation (phase and/or amplitude modulation).
Other work in monolithic integration of some optical receivers have different implementations and/or are still far from being implemented in practical optical systems (see, for example, J. Saulnier et al. “Optical polarization-diversity receiver integrated on Titanium-diffused Lithium Niobate”, IEEE Photonics Technology Letters, v.3. #10, 1991; F. Ghirardi et al. “InP-based 10 Ghz Bandwidth Polarization diversity heterodine photodetector with electro-optical adjustability,” IEEE Photonics Technology Letters, v.6. #7, 1994; D. Hoffman et al. “Integrated optics eight-port 90-degrees hybrid on LiNbO3” Journal of Lightwave technology, v.7. #5, 1989, pp. 794-798).
Accordingly, there is a need for integrated monolithic devices that provide demodulating of quadrature phase shift keying modulated signal (BPSK and/or QPSK) or quadrature amplitude modulated signal (QAM) by use of a single, monolithically integrated device. There is a further need for improved devices that is re-applicable for BPSK and/or QPSK communication systems, for LADAR as well as other remote sensing applications.